Thera and Thrace on Europa

Thera and Thrace are two dark, reddish regions of enigmatic terrain that disrupt the older icy ridged plains on Jupiter's moon Europa. North is toward the top of the mosaic obtained by NASA's Galileo spacecraft. Thera (left) is about 70 kilometers wide by 85 kilometers high (43 by 53 miles) and appears to lie slightly below the level of the surrounding plains. Some bright icy plates which are observed inside appear to be dislodged from the edges of the chaos region. The curved fractures along its boundaries suggest that collapse may have been involved in Thera's formation. In contrast, Thrace (right) is longer, shows a hummocky texture, and appears to stand at or slightly above the older surrounding bright plains. Thrace abuts the gray band Libya Linea to the south and appears to darken Libya. One model for the formation of these and other chaos regions on Europa is complete melt-through of Europa's icy shell from an ocean below.